1. Field of the Invention
The present invention relates to a shock absorber, and more particularly to a shock absorber for a power wheelchair that is suitable for people who are handicapped, elderly or suffering from a disease.
2. Description of Related Art
Wheelchairs were developed for the people who are unable to walk but are otherwise mobile. A power wheelchair is driven by electrical power and is really helpful and convenient to aid people to move about. Power wheelchairs are subjected to shocks when traveling on rough surfaces or bumps on a surface. Shocks cause people sitting in power wheelchairs to feel uncomfortable when the power wheelchairs are moving.
With reference to FIGS. 4 and 5, two conventional shock absorbers (60) are mounted in a power wheelchair (50) to reduce the shock transmitted to a person sitting in the wheelchair (50). The power wheelchair (50) has a pair of front wheels (51), a pair of rear wheels (52) and a power drive assembly (53). The power drive assembly (53) has a drive shaft (531) with two ends (not shown). The ends of the drive shaft (531) respectively extend out of the power drive assembly (53) and are attached to the rear wheels (52). The power drive assembly (53) rotates the rear wheels (52) to move the power wheelchair (50) on a surface.
A conventional shock absorber (60) is mounted on the power wheelchair (50) over the drive shaft (531) near each rear wheel (52) and comprises a suspension spring (61) with a spring stiffness and a suspension rod (62). The suspension spring (61) is mounted around the suspension rod (62). The suspension rod (62) is moveably mounted in the power wheelchair (50) over the drive shaft (531) to move upward when a shock is imparted to the rear wheel (52), and the suspension spring (61) compresses to absorb the shock. While the power wheelchair (50) is moving on a rough or bumpy surface, shocks to the rear wheels (52) will be transmitted to and absorbed by the conventional shock absorbers (60).
However, a conventional shock absorber (60) only has one suspension spring (61) to absorb shocks transmitted from the rear wheel (51). Often, one suspension spring (61) is inadequate to completely absorb the shocks. The shocks will still be transmitted to the power wheelchair (50) where a person is sitting. The suspension spring (61) of the conventional shock absorber (60) compresses to absorb the shocks. However, the compression of the suspension spring (61) creates a restitution force in the suspension spring (61). A sudden shock causes a sudden compression of the suspension spring (61) that generates a sudden restitution force in the suspension spring (61). The restitution force in the spring (61) will immediately be transmitted to the power wheelchair (50) and causes a recoil response in the power wheelchair (50). The person sitting in the power wheelchair (50) will feel uncomfortable.
Furthermore, the suspension spring (61) may be completely compressed by a very heavy person sitting in the power wheelchair (50). With the suspension spring (61) completely compressed, the conventional shock absorber (60) will not be able to absorb any shock because any additional compression of the suspension spring (61) is impossible. In such a state, the convention shock absorber (60) has lost its original capability of absorbing shock.
To overcome the shortcomings, the present invention provides a shock absorber for a power wheelchair to mitigate or obviate the aforementioned problems.